1. Field of the Invention
This invention relates to ultrahigh molecular weight linear polyethylene exhibiting a unique combination of rigidity and toughness in fabricated forms and a process for conversion of normal ultrahigh molecular weight linear polyethylene into fabricated articles having a unique morphological form and exhibiting unexpected rigidity and toughness.
2. Description of the Prior Art
Ultrahigh molecular weight (UHMW) linear polyethylene (LPE) has been known for some time. Such products were frequently obtained following the polymerization process using coordination catalysts (e.g. reaction products of TiCl.sub.4 and aluminum trialkyls) as first disclosed by Professor Karl Ziegler of the Max Planck Institute in Mulheim, West Germany. Such UHMW polyethylenes were not amenable to processing under commercial conditions in the conventional equipment used for plastics fabrication by means of injection molding, blow molding or screw extrusion. Subsequent process developments led to methods for molecular weight control yielding linear polyethylenes suitable for conventional processing above the crystalline melting point (128.degree.-134.degree.C.) and exhibiting normal viscous flow (melt indices of 0.10 to 50 and even higher). Such normal commercial grades usually have a M.sub.w of the order of 150,000.
More recently, means have been disclosed for fabricating UHMW LPE into articles exhibiting outstanding toughness, as measured by impact strength, but relatively low stiffness, due apparently to the relatively low degree of crystallization of the polymers of ultrahigh molecular weight. The properties of sheets fabricated by compression molding from UHMW LPE have been described in some detail. Representative values listed in Table I were disclosed in SPE J, 27, 44 (June 1971).
TABLE I ______________________________________ PHYSICAL PROPERTIES OF UHMW LPE AS CONVENTIONALLY FABRICATED ______________________________________ Density about 0.94 g/cc Tensile Yield Strength 3000-3100 psi. Ultimate Tensile Strength 5500 psi. Ultimate Tensile Elongation 500-900% Tensile Impact Strength 500-1000 ft.lb/in.sup.2 Modulus (in flexure) 85,000 psi. Hardness, Rockwell R. 50 ______________________________________
From the physical properties tabulated in Table I it is apparent that, while UHMW LPE is tough and strong, its density (and hence also its crystallinity) and modulus are more characteristic of branched polyethylenes of intermediate density.
The morphology of linear polyethylenes of normal commercial grades having normal molecular weights (M.sub.w of the order of 150,000) has been extensively studied over the years since first disclosed in the early 1950's. The linear chains exist in a linear zig-zag conformation and crystallize by folding back on themselves to a folded chain (FC) conformation having crystal fold-spacings normally in the range of 100 to 500 A but sometimes, depending on thermal history, over the range of 50-1000 A in the direction of molecular chains.
However, in recent years studies have been disclosed of the effects of very high pressures on the morphology of standard grades of linear polyethylene. Of particular significance in this regard are a series of papers published by Professor Bernhard Wunderlich and associates at Rensselaer Polytechnic Institute [Journal of Polymer Science: Part A-2, 7, 2043-2113 (1969)]. Wunderlich's publications describe the effects resulting from the application of very high pressures [2000-7000 bars (1 bar is approximately 0.9869 atm. or 14.504 lb/in.sup.2)] during either annealing or crystallization from the melt. It is believed that under these conditions initial nucleation into folded chains is followed by chain extension in the solid state to give a new morphological form, termed extended-chain (EC) crystals. Furthermore, crystallization under such super pressures results in a degree of fractionation whereby the lower molecular weight fraction of the macromolecules crystallizes first as bundles of fully extended chains. Subsequently, most of the higher molecular weight fraction also crystallizes into bundles of extended chains or still greater lengths. Some of these higher molecular weight fractions appear to be folded to a reduced extent. However, a typical overall average value for these extended and partially extended chains is 2500 A.
The procedure described by Wunderlich involved placing normal polyethylene in bronze bellows, heating to a temperature above the atmospheric pressure melting point (about 133.degree.C.) and placing the bellows into a heated hydrostatic fluid and applying pressures of 2-7 kilobars over a period of many hours, sometimes with slow cooling during this time. The resultant polyethylenes were characterized as brittle at room temperature, as having a density in the range of 0.980-0.993 g/cc and as having a crystalline melting point in the region of 141.degree.-142.degree.C. The process was repeated generally as described by Wunderlich and it was confirmed that, while the resultant EC polyethylenes had densities in the above range and were very stiff, they were brittle and friable on bending or striking at room temperature and inductile (1-2% elongation at break). Such limitations make them unattractive for use in the plastics industry.